This specification relates to controlling power conversion systems, and more particularly, to mitigating parasitic effects resulting from the activation and deactivation of power conversion systems.
A power conversion system converts power from AC to DC or DC to AC or both. Examples of power conversion systems include inverters, rectifiers, and bidirectional inverters that convert AC to DC in one direction and DC to AC in the opposite direction. Power conversion systems can convert DC power to AC power for delivery to an AC load such as, for example, commercial or residential appliances and the power grid. DC power may be provided to the power conversion system, for example, from a photovoltaic array or a power storage device, such as one or more battery or capacitor banks charged by any combination of power sources, including wind turbines, photovoltaic arrays, hydroelectric generators, and thermal generators. In systems incorporating bidirectional inverters, the power storage device may alternatively or conjunctively receive charge from the power grid.
Despite ongoing efforts to improve their efficiency, power conversion systems generally consume power when they are in an active switching state. Conversion efficiency typically varies with the load, and usually peaks at about two-thirds of the system's capacity (“peak efficiency”). Because power conversion systems consume a relatively fixed amount of power during normal operation, the efficiency decreases when providing lower amounts of power. Some power conversion systems implement standby modes to improve overall efficiency. In typical standby mode implementations, the power conversion system disconnects the isolation transformer from the grid, thereby deactivating the power conversion system and improving the efficiency of the system. This is particularly useful in large photovoltaic array installations where overnight tare (no-load) losses may be significant.
A power conversion system in standby mode is typically reactivated by reconnecting the isolation transformer to the power grid. However, this may result in large in-rush currents, in some cases of up to six times or eight times the rated current of the system. For large scale power systems, the in-rush currents present a significant reactive current draw on the grid, similar to large electric motors or pumps starting up from a stalled state. For example, a typical 100 kW grid-tied inverter may present a 600 kVA load on the grid over a 10 ms period. Simultaneously activating multiple power conversion systems may result in a utility violation due to excessive reactive power draw or complete power system collapse on a small power system.